Methods of obtaining specific operation results without decrypting encrypted numerical values include a method called secret calculation (e.g., Non-patent literature 1). The method of Non-patent literature 1 performs encryption that allows three secret calculation devices to share the fragments of the numerical value, and the three secret calculation devices perform cooperative operation, which can allow the three secret calculation devices to hold the results of addition and subtraction, constant addition, multiplication, constant multiplication, logical operation (negation, logical conjunction, logical disjunction, and exclusive OR), and data format conversion (integer, and binary numeral) without decrypting the numerical value, in a state of being shared among these devices, i.e., being left encrypted.
There is a method of Non-patent literature 2 as a method capable of efficiently performing multiplication using the secure texts of plain texts “a”, “b” and “c” having a relationship c=ab in a case of multiplication through secret calculation.